Optical images from telescopes may be transformed into electrical images by the utilization of mosaic focal planes incorporating a number of photo sensitive detectors, such as infrared or visual photo diodes. For mosaic focal planes, cost increases with increasing numbers of detectors. For a given area the number of detectors and hence the cost will increase with desired spatial resolution. Another problem with mosaic focal planes resides in the difficulty of obtaining sharp resolution over the entire focal plane. This is due to the fact that the sharpest resolution in a focal plane tends to be in the center.
Still further, large mosaic focal planes generally include millions of pixels requiring extensive computer processing which is an expensive proposition and quite time consuming. One could use the concept of a high spatial resolution portion of the focal plane surrounded by a low resolution outer ring. However, in order to move the high resolution portion to another area it would be necessary to tilt the whole sensor to another pointing direction and also to have extra detectors on the edge of the outer ring to accommodate the new position of the image. This concept has the difficulty of also requiring extra detectors. For very large sensors, the telescope cannot be quickly moved from one pointing direction to another because it would vibrate and cause a blurred image.